Solar cells, that is, photovoltaic cells which are the most advanced technology among clean energy generation systems with the potential to be used with few limitations are recently on a trend of becoming wide-spread throughout industry as well as daily life in Germany, the United States, Japan, or the like, since the 2000s. At present, mono/polycrystalline silicon photovoltaic cells are most generally used, however, these entail significant costs for production and installation compared to fossil fuel facilities such as gas, petroleum, etc., and such high costs are considered to be a high entry barrier. In order to solve this problem, attempts to develop various photovoltaic cells without using silicon (i.e., an inorganic thin film photovoltaic cell, a fuel-sensitive photovoltaic cell, an organic photovoltaic cell, a quantum dot photovoltaic cell, etc.) have been continued.
Chlorophylls are a key component of all livings existing through photosynthesis to produce nutrients using solar light and water in a natural system, such as plants, green algae, reddish bacteria, or the like, which are pigments serving as a kind of antenna to absorb light energy. These chlorophylls include reaction centers of photosystems II and I, and are a series of cofactors having a unified mechanism wherein light harvesting, energy transfer and electron donating take place continuously. Physical and chemical properties of chlorophylls have been determined indicating that they have various structures by the use of tetrapyrrole templates.
Chlorophylls have excellent absorbency in a visible light region to thus efficiently utilize solar energy incident upon the earth's surface. Also, since chlorophylls are easily extracted in great quantities from plants or green algae, these may become a solution to noticeably reduce a unit cost of a photovoltaic cell if used as a photo sensitizer of the photovoltaic cell. However, a method of efficiently aligning and combining a photovoltaic cell device formed of existing inorganic materials and chlorophylls as a natural biomaterial has not been yet proposed, and hence currently results in a very low light energy transfer efficiency. Accordingly, there have been difficulties in the development of a photovoltaic cell using chlorophylls.
Graminoids are often found in grasses abundantly available in tropical and subtropical areas, and contain both of chlorophyll molecules and graminone lignan so as to exhibit excellent absorbency and photo-electron generation performance, compared to existing chlorophylls. Since photo sensitizer molecules in such a natural system generate photocurrent at an average of about 10 pA per molecule, improvement of photo-electron generation characteristics is required for commonly using these molecules. The surface plasmon effect refers to a characteristic in that, if a rare-metal such as silver or gold exists in a size of several nanometers, this metal may be similar to a semiconductor photo-catalyst material, absorbing light and scattering light in a constant wavelength range again. Therefore, when the above metal is suitably mixed with the bio-photo sensitizer and used, the photo-electron generation efficiency may be increased by about 10 times or more while not destroying any bio-material. Further, existing bio-photovoltaic cells involve poor interfacial contact between the photo sensitizer and a semiconductor-based electron acceptor, hence causing most of electrons generated in the photo sensitizer at an interface therebetween to become extinct due to re-coupling of electrons-holes. Accordingly, this drawback must also be improved upon.
Attempts to use a naturally extracted photo sensitizer as a source of photo-electron generation for a photovoltaic cell have been continuously investigated since research into application of a photosynthetic reactor center extracted from Rhodobacter sphaeroides as one of photosynthetic bacteria as well as chlorophylls extracted from spinach to a semiconductor device was disclosed by R. Das et al. (2004). However, since the wavelength range of light absorbed by natural extracts is very narrow and, due to significant differences in environment from when these exist in a natural state inside a living object, a route of flowing electrons and holes generated by receiving light is discontinued, re-coupling of electrons and holes often occurs whereas coupling between the natural extract and the semiconductor-based electron acceptor does not take place actively. Therefore, an amount of finally collected photo-electrons is very small and a bio-photovoltaic cell exhibits considerably reduced light-energy conversion efficiency. The highest light-energy conversion efficiency reported until now is 1.51% only in a case of using a photo sensitizer extracted from sea tangle.
Accordingly, the present invention proposes a method for fabricating a high efficiency bio-photovoltaic cell by using silver nanoparticles expressing surface plasmon effects, a sheet type semiconductor electron acceptor facilitating active bonding between different materials and an organic ligand material, so as to overcome the problems described above.